The present invention relates to a teletext receiver, equipped with means permitting an early acquisition of certain transmitted articles.
In order to avoid any confusion regarding the terminology to be used hereinafter, it is firstly pointed out that the expression videography designates a telecommunications process making it possible to provide a user with alphanumeric or graphic messages on a display screen. In a variant of this process, the messages are systematically transmitted by a television network and the user can make a choice between these messages. It then constituted so-called transmitted videography, which is also called teletext and in France, the "ANTIOPE" system is in accordance with this variant.
In a teletext system, the information is coded in digital form and it is organised in accordance with the French DIDON standard (for digital data transmission). This standard is described in numerous documents, particularly in No. 3.80 and 1.82 of the Journal "Radiodiffusion-Television", in French Pat. No. 2,313,825 published on June 6, 1975, in its first certificate of addition 2,393,480, published on Mar. 6, 1977 and in French Pat. No. 2,404,350, published on Sept. 26, 1977.
In connection with this standard, it is merely pointed out that the digital data are organised into bytes grouped in a block. A heading, which is also formed from bytes, is added to each block and the combination constitutes a packet. For use in France in accordance with the so-called L standard, the maximum size of a packet is 40 bytes. Such a packet is inserted in a line of the image, either solely in that part of the image reserved for the vertical scanning return and partly occupied by the frame sync signals and possibly other control signals, the remainder of the image being occupied by conventional video signals, or over the complete image in the absence of any video signal. On reception, the packets are sorted by demultiplexing and, by heading processing, the data blocks are restored.
In the ANTIOPE system, separate messages can be multiplexed within the same digital channel by means of structuring in the form of "articles". The structure of the articles makes it possible to multiplex together, either the screen description messages, or other types of information.
Each article is constituted by four consecutive sequences: an article start, an article heading, a data field and an article end. Such a structure is shown in FIG. 1.
The article start and end are designated by special codes. In general terms, it is known that the codes used in teletext are arranged in a table having 16 rows and 8 columns, each code being designated by the rank or order of its column (expressed from 0 to 7) and the rank or order of its row (expressed from 0 to 15). Thus, for the article start use is made of two SOH codes of ranks 0/1 and RS of ranks 1/14. The article end is coded by ETX-EOT of respective ranks 0/3 and 0/4.
The article heading is constituted by a sequence of bytes, which have undergone Hamming coding (i.e. per byte: 4 information bits and 4 protection bits). It consists of the 3 following successive parts: a classification sequence, a connecting byte and an interpretation sequence.
The classification sequence makes it possible to transmit a "classification number" designating the message in the file of messages to be transmitted and participates in the linking of the source and receiver. For example, it consists of the selection of a "page" by its "number". This sequence has three bytes C1, C2, C3 (indicated in their transmission order).
Each byte has a value, which can be hexadecimally notated. The 16 possible values are designated 0 to 9 for the first 10 values and A, B, C, D, E, F for the 6 last values. The values assumed by these bytes define a first classification of the nature of the messages carried by the article.
The different articles are distributed into three classes, in accordance with their classsification number.
First class: C1=C2=C3=0: this article number is reserved for the description of the "rows 0", row 0 being "an auxiliary screen" intended for the display of service messages. In teletext, a reception terminal is supposed to acquire and interpret all the articles of number 0 (the interpretation of a given article corresponding to an explicit request from the user having a priority, i.e. it hides the acquisition of the rows 0 during processing.) PA1 Second Class: C1C2C3 between 001 and 999. This numbering field is reserved for the transmission of information relating to a particular displayable message relating to the main screen (rows 1 to 24 and margins). In this case, C1, C2 and C3 respectively represent hundreds, tens and units of the page number. PA1 Third class: At least one the bytes C1, C2, C3 is equal to A, the others being between 0 and 9. The value A must be considered as equivalent to any random value taken by the corresponding byte between 0 and 9. However, the number 000 must not be considered as included in this equivalence. This mechanism makes it possible to address a single message in a simultaneous manner relative to several terminals connected to "numbers" of different pages. PA1 so-called service pages which must be systematically acquired and therein is generally found information relating to the selected article (title, magazine number, etc) indicating the origin of the magazine (distribution chain, data bank name, etc); PA1 so-called "telecharging" or "teleloading" units which are used for transmitting information common to several user pages, said units being possibly alphabets, colour ranges, page bottom information; PA1 message units, which can be information used by the terminal for controlling the starting up of equipment such as magnetoscopes and can also be summary-type information on the transmitted or distributed magazines; PA1 optionally instructions intended for the terminal (e.g. an instruction to display a subtitle); PA1 entry in session units which contain information defining the link between the articles, e.g. one unit will contain the list of articles in the magazine and for each article the associated telecharging page numbers. PA1 Fourth class: At least one of the bytes C1, C2, C3 has a value between B and F. It is possible to distinguish the following subclasses. If C1=B: The corresponding values are reserved for articles which are only selected when the decoder is in an appropriate state (television, subtitling, programme identification investigation, etc), other than that in which the user explicitly or implicitly requests a page of the first category (0 to 999);
All these questions are described in detail in 3.80 and 1.82 of the Journal "Radiodiffusion-television".
Having recalled these various characteristics of a teletext system, reference will now be made to the problem to be solved by the invention.
A magazine intended for a teletext system is distributed or transmitted in a cyclic manner. The articles forming this magazine (whose structure has been defined) are also cyclically transmitted. On reception, the user selects an article by typing on a keyboard a number having three digits between 001 and 999. On the basis of what has been stated hereinbefore, it is clear that the user only has access to the articles of the second class, the number displayed on the keyboard coinciding with the number of a page in the magazine.
In the first teletext systems, the information necessary for the display of a page was entirely contained in the corresponding article. The choice of a single page from among the cyclically transmitted pages was brought about by appropriate software making it possible to compare the codes C1, C2, C3 of the articles received with the three digits selected by the user by means of this keyboard.
However, nowadays the systems are much more complex and the information corresponding to a page to be displayed can no longer be included within a single article.
Thus, different articles or units have been made to appear, such as e.g.:
Each of these articles is given its own number. Thus, a teletext receiver must take into account several articles for a single requested page.
The tests to be carried out may consequently take a considerable time, so that at the transmission source it is necessary to bring about a greater spacing between the articles, so that the receiver can check the headings of all the magazine articles. It is therefore necessary to use the transmission channel in a non-optimum manner.